This invention relates generally to fuel cells and more specifically to fuel cell protection means.
Fuel cells provide clean, direct current (DC) electricity. Fuel cells convert reactants, namely fuel and oxidant (air or oxygen), to generate electric power and reaction products. A typical fuel cell power source is constructed from a stack of cells coupled in series as shown in FIG. 1. For a series electrical connection 100, the same amount of current is drawn from each cell 102. In practice, each cell 102 has slightly different performance characteristics.
Fuel cells exhibit a decreasing output voltage as the current output is increased. This curve is not linear and tends to drop off faster at higher currents. Consequently, as can be seen in FIG. 2, the power vs. current characteristic 200 generally has a peak 202 at mid-current levels and then rolls off. Below the xe2x80x9cpeak powerxe2x80x9d point 202, as more current is drawn, the power increases. Above the xe2x80x9cpeak powerxe2x80x9d point 202, as more current is drawn, the power decreases. The region in which the slope of power vs. current is negative is referred to as negative dP/dI region 204. Beyond point 202, if the system tries to draw more power from the fuel cell, it quickly goes to low power (low voltage) and typically the support circuitry shuts off and the whole system stops working. In some cases, one of the cells may fail to work properly because of a temporary problem within the cell, such as a blocked micro-channel caused by particles or carbon dioxide bubbles. Therefore, it is desirable to monitor the system operation and avoid this condition.
Prior art fuel cell systems have included systems that monitor and compare the fuel cell voltage to a reference fuel cell voltage (Vfc), activating an alarm when the Vfc is exceeded. However, this type of system only provides for monitoring and alarm activation. Other systems have utilized a high power resistor and thermistor in conjunction with monitoring a voltage, and still other systems have used optoisolators. Again, these systems focus on monitoring the cell voltage and lack any constructive means of circuit protection.
Accordingly, a need exists for a protection apparatus and technique to prevent any cells from operating in the negative dP/dI region.